<p>A cooling system is incorporated into an engine cooling circuit including a pump for circulating a coolant of an engine and a radiator for cooling the coolant of the engine. The cooling system includes a first passage portion provided between the engine and a coolant outlet of the pump; a second passage portion provided between the engine and an coolant inlet of the pump, a rotary valve disc interposed in the passage portions and being rotatable so as to simultaneously control the coolant flowing through the first passage portion and the coolant flowing through the second passage portion.</p>